1. Field of the Invention
The present invention relates to the field of communications. In particular, the present invention provides an improved apparatus and method for determining channel quality based on evaluation of a control channel.
2. Background of the Invention
Wireless communication systems continue to experience rapid growth to meet the needs of cellular telephone users. The demand for wireless communication has increased the number of users attempting to gain access to the system and has placed higher expectations on the clarity provided by wireless communication systems. For example, users prefer the audio quality of their wireless service to meet that of a wired connection, and expect wireless service to consistently place a call. Furthermore, users expect the number of xe2x80x9cdroppedxe2x80x9d calls to be at a minimum.
The expanding wireless systems places a premium on existing cellular sites, and in particular, the available bandwidth or bit rate capacity of each base station. The limited number of suitable locations at which cellular sites may be placed increases the demands on existing cellular base station capacity thereby placing an even greater premium on the bandwidth or bit rates of existing base stations.
In general, each cellular site operating in current digital wireless communication systems, such as for example, GSM (Global System for Mobile Communications) use at least two channels over which communication between a mobile station and a base station (cellular site) may occurxe2x80x94at least one data or traffic channel and of at least one control channel. The data or traffic channel shoulders the majority of the exchange by carrying the encoded voice or data transmitted between mobile and base stations of the wireless system.
The control channel provides a communication link between the mobile station and the base station for mobile management tasks. Examples of data exchanged over the one or more control channels include mobile station location, in-coming call traffic control, billing information, and security verifications. Similarly, control channels often provide paging or broadcast services. The amount of data exchanged over the control channel is relatively insignificant compared to the amount of data exchanged over the traffic channel. However, the control channel is very robust because of the importance of the control channel. Although not the focus of the present invention, these safeguards may include extensive redundancy in the form of block-coding and convolutional coding. The extensive redundancy associated with the control channel provides robust error detection and correction.
A mobile station is typically configured with a fixed bandwidth or bit rate. The communication system assigns the total bit rate based on a system standard. This total bit rate does not change regardless of channel quality, and is typically selected for worst case or near-worst-case channel conditions. Accordingly, the maximum number of users per base station is also fixed.
Various methods exist for determining the quality of the traffic channel. The typical methods include traffic channel frame success rate, signal strength, bit error rate on the traffic channel, and the frame ratio. However, the traffic channel provides an unreliable channel quality indicator due to its high incidence of noise. For example, noise or static over the traffic channel almost always exists. Likewise, a plurality of interference generating sources exist, such as a passing truck, an underpass, or other electromagnetic interference which are inherent in wireless communication. The high incidence of interference on the traffic channel coupled with a generally small degree of error correction (redundancy) on the traffic channel hinders its employ as a reliable channel quality indicator.
The inventors recognized the need for a more reliable and accurate method of evaluating the quality of the traffic channel to allow more flexibility in the system, such as enabling modifying the channel coding and/or permitting a higher bit or lower bit rate.
The present invention provides a method and apparatus for use in a digital communication systems, such as GSM, to more accurately evaluate the traffic channel quality, and in response, modify the channel coding used during the transmission of speech data or modify the channel coding and/or the effective bit rate on a traffic channel.
The present invention provides an improved method and apparatus for evaluating the traffic channel quality. An associated control channel is present during an active speech call or active data transmission. Because the control channel is very robust, the control channel quality is very predictive of the actual channel quality of the traffic channel where the traffic channel and the control channel are related, such as by frequency, or otherwise experiencing similar interference and fading. In other words, the efficacy of a channel quality determination based on the control channel is reliable because on the control channel spurious noise is less likely to provide a non-reliable indicator of steady-state changing channel quality. During an active voice or data call, the associated control channel, such as the Slow Associated Control Channel in a GSM communication system, is monitored to obtain an estimation of gross bit error rate (BER). The BER of the control channel is calculated by reencoding the successfully decoded information bits and comparing them to the received bits. In a preferred embodiment, BER rate is only calculated if a PASS result from block decoding. The estimated BER, which may be filtered, is then used to determine the control channel quality. The quality of the control channel can then be applied to the traffic channel and used to select channel coding for the traffic channel which maximizes the efficient use of system bit rates without significantly degrading the speech quality. The determination of the traffic channel quality from the control channel estimated BER can be made by preset thresholds or variable thresholds. Where the control channel and traffic channel are related such that the control channel experiences similar interference to the traffic channel, as in most cases, the control channel signal quality is predictive of the traffic channel quality.
In one embodiment, the invention comprises a method for estimating the quality of a traffic channel in a wireless communication system having at least one control channel and at least one associated traffic channel. In particular, the method involves receiving a coded version of a control channel signal, decoding and re-encoding the control channel signal, and comparing the received coded version of the control channel signal with the decoded (corrected) and re-encoded control channel signal to determine the difference between the two. The difference represents errors in the control channel signal, and is used to calculate the bit error rate. Based on the difference, a similar signal quality is assumed for the traffic channel.
In another embodiment, the channel quality estimator comprises a signal decoder configured to decode an encoded control channel signal and a signal storage device configured to store the received control channel signal while the estimator re-encodes the decoded control channel signal. A comparator coupled to the re-encoder and the signal storage device determines the number of bits which differ between the two. The difference, or bit error rate, enters an evaluation module which estimates the quality of the channel based on the bit error rate.
In yet another embodiment, the invention comprises an error correction controller configured to select the error correction overhead (channel coding) to apply to signals on a traffic channel. The controller comprises a signal decoder configured to decode an encoded control channel signal. A re-encoding module re-encodes the decoded (corrected) signal, and a comparator configured to receive both of the re-encoded signal from the signal decoder and a stored signal from a signal storage device (containing the received signal) compares the two signals. An error correction selector receives information from the comparator and evaluates the control channel signal quality. Based on the control channel signal quality, the selector selects appropriate overhead (error detection and correction coding) to apply to outgoing data transmissions on the traffic channel.
In one embodiment, the frame error rate can be used rather than the bit error rate. As with the bit error rate, preferably, the decoded control channel signal is only re-encoded and the frame error rate calculated if a PASS results from block decoding.